SIP retry on 503/isdn 63 release code

if a call is being made originating from the PBX, towards the SBC end, and for some reason the SBC replies to the 5400 with "503 unspecified/service unavailable", the 5400 instead of dropping the call keeps retrying by sending new SIP invites to the SBC, until the caller hangs up. How do i prevent this? i.e. how do i force the 5400 to send a user busy/fast-busy response to the caller (or terminate the call anyway) after 2 attempts?

i can't find a command regarding this except for "no voice hunt" (which doesn't seem to do the trick)